1. Field of the Invention
This invention generally relates to a process for the reclamation of constituents contained in common industrial waste streams including essentially pure metal oxides and metals. This invention more specifically relates to a method for the recovery of essentially pure zinc oxide and other compounds such as lead compounds, copper compounds, silver compounds, and cadmium compounds from electric arc furnace (EAF) dust emanating from the steel industries processes.
2. Description of Related Art
Metallurgical processes, such as steel processes, result in waste by-products of iron and steel dust. There have been many attempts to recover valuable metal and chemical values such as zinc, lead, cadmium, silver, and copper in this dust and to obtain a by-product or final waste material that can be recycled or disposed of with minimal metal leaching problems.
Zinc oxide typically is a fine white or grayish powder that has a variety of uses including as a rubber accelerator, as a pigment, as a dietary supplement and in the semiconductor field. Zinc oxide is found in commercial by-products including waste material streams such as fly ash and flue dust. Methods for recovering zinc oxides are known in the art, including recovering zinc oxide from industrial waste materials. Such previous methods have included leaching with mineral acid, caustic soda, ammonium hydroxide, and ammonium carbonate solutions. However, these methods have low yields of zinc oxide and typically do not recover pure zinc oxide, the recovered zinc oxide being contaminated with other metal salts. Therefore, in order to obtain pure zinc oxide, subsequent reduction and washing processes were necessary.
One such method for recovering zinc and lead is the Waelz kiln. The Waelz kiln uses a directly heated counterflow rotary kiln to roast the materials under reducing conditions. The reducing roasting process comprises directly heating and roasting the iron and steel dust in the Waelz kiln in a reducing atmosphere under suitably selected conditions of temperature and retention time, thereby separating zinc and lead through volatilization from the dust and enabling iron to be discharged in the form of solid direct reduced iron. U.S. Pat. No. 4,525,208 to Yasukawa attempts to avoid perceived problems with the Waelz kiln caused by the depositing of material on the walls of the rotary kiln by running the volatilization in two stages. In the first stage the material is heated and zinc and lead are partially evaporated at a lower temperature in a rotary kiln. In the second stage, the solid material from the rotary kiln is continuously fed into the rotary smelting furnace where fluxes are added to the material to lower the melting point facilitating the evaporation of the metals from the molten stage.
U.S. Pat. No. 3,850,613 to Allen discloses a method for the treatment of steel mill waste dusts containing zinc. The Allen ""613 method includes briquetting the dust with carbon to reduce the zinc and lead oxides to the zinc and lead metals and then volatizing the zinc and lead at high temperatures by heating the briquettes to 1800xc2x0 F. to 2500xc2x0 F., and then oxidized to ZnO and PbO in the gaseous phase. The ZnO and PbO then are recovered, with the remainder of the material used as the charge to a steel-making furnace.
U.S. Pat. No. 5,013,532 to Sresty discloses a method for recycling electric arc furnace dust. The Sresty ""532 method for reducing the zinc contained in an EAF dust, volatilizing the metallic zinc so produced from the mass of the dust, and reoxidizing the metallic zinc to zinc oxide along with the simultaneous regeneration of hydrogen that can be recycled to treat additional EAF dust. The Sresty ""532 process involves heating a raw material containing a mixture of metals and metal oxides in briquette, pellet, granular or lump form in a furnace to about 900xc2x0 C. to about 1200xc2x0 C., contracting the heated raw materials with a stream of hydrogen gas to reduce and vaporize the desired metal oxide or oxides, humidifying the stream of hydrogen gas and metallic vapors recovered from the furnace with water in the form of water vapor or steam to lower the temperature of the gas stream to about 700xc2x0 C. to about 900xc2x0 C. so that the metallic vapors react with the water to produce a solid metallic oxide or mixture of metallic oxides, humidifying again to a temperature in the range of about 150xc2x0 C. to about 250xc2x0 C., separating the solidified metal oxide from the hydrogen gas stream, condensing excess water from the hydrogen gas stream, and recycling the hydrogen gas stream.
U.S. Pat. No. 3,262,771 to Ban discloses a system for the recovery of steel and zinc from waste materials using a molten stage, including an electric arc furnace, an electrothermic smelting furnace and a slag fuming method. In the Ban ""771 process, the recovery of zinc and lead is carried out in a molten stage by using an electric arc furnace. Zinc oxide is recovered as an overhead vapor, and iron is recovered as a liquid.
U.S. Pat. No. 5,188,658 Aune also discloses a method for the recovery of zinc from zinc-containing waste materials using a molten stage, including an electric arc furnace, an electrothermic smelting furnace and a slag fuming method. In the Aune ""658 process, zinc and lead are recovered from the molten stage in an electrothermic smelting furnace. The electrothermic smelting furnace described in the Aune ""658 patent requires that the furnace be kept at high temperatures in order to maintain a volume ratio between CO2 and CO in the gas atmosphere in the smelting furnaces below 0.3.
U.S. Pat. No. 3,017,261 to Lumsden also discloses a method for the recovery of zinc from zinc-containing waste materials using a molten stage, including an electric arc furnace, an electrothermic smelting furnace and a slag fuming method. In the Lumsden ""261 patent, zinc and lead are recovered from the molten stage with a slag fuming method using a stationary furnace where metals are volatilized by melting the iron and steel dust completely and blowing air and reducing agent such as coal or coke into the molten iron.
U.S. Pat. No. 4,840,671 to Lynn discloses a process to render such industrial waste non-hazardous without recovering zinc or lead. In the Lynn ""671 process, electric arc furnace dust, steel dust from the production of certain specialty grades of steel, is rendered less hazardous by complexing the dust in a lime kiln dust, fly ash and hydrated lime mixture and then adding an aqueous solution containing ferrous hydroxide and calcium sulfate. The Lynn ""671 process is based on the pozzolanic reaction of materials containing anhydrous alumino-silicates that, in the presence of lime, water and chemicals, adsorb and/or physically entrap the heavy metals present in EAF dust into a calcium-alumino-silicate matrix.
U.S. Pat. No. 3,849,121 to Burrows discloses a method for the selective recovery of zinc oxide from industrial waste. The Burrows ""121 method comprises leaching a waste material with an ammonium chloride solution at elevated temperatures, separating iron from solution, treating the solution with zinc metal and cooling the solution to precipitate zinc oxide. The Burrows ""121 patent discloses a method to take EAF dust that is mainly a mixture of iron and zinc oxides and, in a series of steps, to separate out the iron oxides and waste metals. However, the material obtained in the last step is a mixture of a small amount of zinc oxide, hydrated zinc phases that can include hydrates of zinc oxide and zinc hydroxide, as well as other phases and a large amount of diamino zinc dichloride Zn(NH3)2Cl2 or other similar compounds containing zinc and chlorine ions. Currently, the Burrows ""121 method is not economically viable because of Environmental Protection Agency guidelines established subsequent to the issuance of the Burrows patent. Additionally, the Burrows ""121 method is not a continuous method and, therefore, is not economical as a continuous process.
U.S. Pat. No. 4,071,357 to Peters discloses a method for recovering metal values which includes a steam distillation step and a calcining step to precipitate zinc carbonate and to convert the zinc carbonate to zinc oxide, respectively. Peters ""357 further discloses the use of a solution containing approximately equal amounts of ammonia and carbon dioxide to leach the flue dust at room temperature, resulting in the extraction of only about half of the zinc in the dust, almost 7% of the iron, less than 5% of the lead, and less than half of the cadmium. Steam distillation is contrary to dilution. Steam distillation precipitates zinc carbonate, other carbonates and iron impurities. Steam distillation also disadvantageously results in an increase in temperature that drives off ammonia and carbon dioxide, resulting in the precipitation of iron impurities and then zinc carbonate and other dissolved metals. The purity of the zinc carbonate obtained depends on the rate of steam distillation and the efficiency of solids separation as a function of time. Calcining converts the zinc carbonate to zinc oxide, whereas washing and drying at temperatures between 100xc2x0 C. and 200xc2x0 C. converts the zinc compounds to zinc oxide.
U.S. Pat. No. 5,464,596 to Myerson, commonly assigned with the present application, discloses a method for the recovery of zinc oxide by treating a waste stream with a 23% ammonium chloride at 90xc2x0 C., separating undissolved components from the solution, displacing undesired metal ions from the solution using zinc metal, cooling the solution to precipitate out zinc compounds, washing the precipitate to remove various soluble zinc compounds, leaving zinc oxide of greater than 99%. The Myerson ""596 patent teaches that ammonium chloride solutions must be at least 90xc2x0 C. to sufficiently dissolve the zinc compounds. Heating an aqueous solution to such a temperature requires the expenditure of large amounts of energy. It is further taught that while NH4Cl concentrations below 23% do not dissolve the maximum amount of zinc oxide from the waste material, concentrations greater than 23% result in an impure zinc oxide due to the tendency of the NH4Cl to precipitate out of solution with the zinc compounds at such high concentrations. Furthermore, the cooling of the product solution results in the precipitation of various zinc species, resulting in crystallization of some species. Because of this, using the cooling step disclosed in Myerson ""596, one cannot use concentrations of ammonium chloride solutions above about 23%, limiting the usefulness of the process disclosed in Myerson ""596. Further, contaminates of the zinc oxide must be removed by an additional washing step.
U.S. Pat. No. 5,759,503 to Myerson, et al., commonly assigned with the present application, discloses a method for the recovery of zinc oxide by dissolving zinc oxide in an intermediate, diluting the intermediate by a factor of 3 to 30 by adding 70-100xc2x0 C. water, and filtering out the resultant zinc oxide crystals. Myerson ""503, along with its family of patents, discloses using ammonium chloride solutions of 23% and teach that using higher concentration ammonium chloride solutions will produce undesired results. This was the understanding at the time of invention of the processes disclosed and claimed in the Myerson ""503 patent and its family of patents. These undesired results include the precipitation of various zinc species, resulting in crystallization of some species. Because of this, using the cooling step disclosed in Myerson ""596, one cannot use concentrations of ammonium chloride solutions above about 23%, limiting the usefulness of the process disclosed in Myerson ""503. Although the processes disclosed and claimed in Myerson ""503 patent are valuable and perform admirably, it has now been discovered that by altering the process steps, including the addition of a dilution step, ammonium chloride solutions of between 23% and 30%, and of 30% or greater, can be used to recycle industrial waste streams.
Further, Myerson ""503 differs from the present invention in that concentrations above 23% are not disclosed and, in fact, were specifically discouraged because, at that time and using those process steps, using ammonium concentrations above 23% did not result in a suitable process because concentrations of ammonium chloride above about 23% tend to precipitate out ammonium chloride along with the zinc oxide when the solution is cooled. It is undesirable to have the ammonium chloride precipitate out. This problem is solved by the present invention.
In general, the Myerson family of patents specifically discloses methods for the recovery of zinc oxide comprising the steps of treating a waste stream with a 23% ammonium chloride solution and separating undissolved components from the solution. It is further taught that NH4Cl concentrations greater than 23% result in an impure zinc oxide due to the tendency of the NH4Cl to precipitate out of solution with the zinc compounds at such high concentrations. Furthermore, the cooling of the product solution results in the precipitation of various zinc species, resulting in the unwanted crystallization of some species. Because of this, prior to the present invention, the inventor and those skilled in the art were under the belief that one could not use concentrations of ammonium chloride solutions above about 23%, thus limiting the usefulness of the process disclosed in the prior Myerson patents.
Therefore, there exists a need for an alternative method that will recover essentially pure zinc oxide from industrial waste materials that is economical, quick, and efficient and, optionally also will allow the recovery of elemental lead, cadmium, and copper from industrial waste materials, at the lower end of the temperature range and at higher leach solution concentrations than previously thought possible.
The present invention satisfies these needs in a method that recovers constituents contained in common industrial waste streams from, for example, the metals industries. Many of these industrial waste streams contain zinc, lead, copper, silver, and/or cadmium compounds, all of which can have a commercial value, or which are undesirable waste products. For example, one commercially valuable material that can be recovered is essentially pure zinc oxide from industrial waste material streams containing zinc or zinc oxide. Along with the essentially pure zinc oxide, zinc metal also can be recovered, as well as values of other metallic elements originally contained in the waste material, such as lead, silver, and cadmium. The solutions used in the process are recycled such that liquid wastes are kept to a minimum and, ideally, eliminated. The solids recovered from the process, namely, the chemical and metal values, and other residues all can be used in other processes. One such residue, an iron oxide cake, is of such a quality that it can be used directly as the feedstock for the typical steel production process.
Briefly, the present invention provides a method for recovery of high purity chemical and/or metal values from industrial waste streams such as electric arc furnace effluents (dusts, fumes, and vapors) containing the values or compounds containing the values (such as zinc compounds) comprising the steps of: (a) leaching the waste stream with an ammonium chloride solution at a temperature of at least about 70xc2x0 C., resulting in a first product solution and undissolved materials; (b) adding a displacing element of compound (such as zinc metal) to the first product solution in a cementation step, whereby displaceable ions contained in the first product solution are displaced by the displacing element or compound and precipitate out of the first product solution, generally as metals, leaving a second product solution; and (c) diluting the second product solution with water, resulting in the precipitation of the desired high purity value (such as zinc oxide) and a third product solution. The dilution step circumvents the crystallization of various species, allowing the use of the higher concentration ammonium chloride solution of approximately 23% or greater, and preferably 30% or greater, by weight. The various undissolved precipitates produced during the process comprise both waste products and chemical and metal values that can be recovered and sold, used in subsequent processes, or added to the feed to various industrial processes such as the iron and steel making processes.
The third product solution is concentrated after removing the desired high purity value (such as zinc oxide), resulting in a fourth product solution comprising greater than 30% ammonium chloride. The fourth product solution then is combined with the original ammonium chloride solution of Step (a) to leach the industrial waste stream in a continuous process.
If the industrial waste material stream contains significant amounts of iron, the waste stream preferably is heated in a reducing atmosphere prior to leaching, resulting in an iron-containing residue and a combustion product (dust, fumes, and/or vapors) waste stream comprising zinc oxide. The dust, fumes and/or vapors then are subjected to the process disclosed in this specification.
In an extension of the present invention, using a waste stream such as typical electric arc furnace dust (which typically contains zinc, cadmium, copper, lead, and iron metals and compounds) the precipitated metals will comprise zinc, lead, cadmium and copper compounds. This alternate embodiment further comprises the additional steps of: (1) treating the precipitated metals with an aqueous solution of either H2SO4 or NH4SO4, whereby zinc, cadmium, and copper compounds go into solution and lead compounds do not, resulting in a fifth product solution comprising zinc, cadmium and copper compounds and a second undissolved precipitate comprising lead compounds; (2) adding zinc metal to the fifth product solution, whereby cadmium and copper compounds are displaced by the zinc metal and precipitate out of the fifth product solution as a third precipitate, leaving a sixth product solution; and (3) treating the sixth product solution with calcium chloride, resulting in the precipitation of CaSO4 from the sixth product solution. Again, the undissolved precipitates produced during the process comprise both waste products and chemical and metal values that can be recovered and sold, used in subsequent processes, or added to the feed to various industrial processes such as the iron and steel making processes.
If it is desired to control sodium or potassium chloride concentrations during the process, additional steps of: (i) adjusting the pH of the second product solution to between about 5 and about 8 prior to step(c); (ii) cooling at least a portion of the second product solution to precipitate diamino zinc dichloride; (iii) evaporating the second product solution to precipitate NaCl and KCl; and (iv) combining the second product solution with the ammonium chloride solution to leach the waste stream. Typically, the second product solution is acidic and a base is added to adjust the pH of the second product solution. However, if the second product solution is basic, an acid is added to adjust the pH of the second product solution. In any event, a base, an acid or a combination can be added to the second product solution to achieve the desired pH of between 5 and 8. Suitable bases include NH4OH, NaOH, KOH, and Ca(OH)2. Suitable acids include hydrochloric acid, acetic acid and nitric acid.
Therefore, it is an object of the present invention to provide a method for recovering chemical and/or metal values from industrial waste material streams, such as fly ash or flue dust, which contain other metals and/or compounds, such as iron oxide, zinc, zinc oxide, lead oxide, cadmium, copper and other materials.
It is another object of the present invention to provide a method for recovering zinc oxide from waste materials, such as fly ash or flue dust, which contain other metals, such as iron oxide, lead oxide, cadmium, copper and other materials.
Yet another object of the present invention is to provide a method for recovering zinc oxide in which all leaching and washing solutions are recycled for further use, and no leaching or washing solutions are disposed of into the sewers or the environment.
Still another object of the present invention is to provide a method for recovering zinc oxide that also results in the precipitation in elemental form of any lead, cadmium and copper metals contained in the starting materials.
Another object of the present invention is to provide a method for recovering zinc metal, zinc oxide and/or iron oxide that is economical, quick and efficient.
These objects and other objects, features and advantages of the present invention will become apparent to one skilled in the art when the following Detailed Description of the Preferred Embodiments is read in conjunction with the attached figures.